Known in the art, for example from the German Offenlegungsschrift 25 59 430, is an apparatus for testing identification cards or the like and employing for this purpose a photodiode. In order to prove the authenticity of the documents inserted into the verifier, use is made of special test elements constructed of compounds of rare earth elements which have the property to reflect or transmit impinging light with certain delays. The known apparatus exploits the delay characteristics of these elements for recognizing the authenticity of the inserted document. The known apparatus employs a pulse oscillator whose output signals pass through the special test elements and are then fed to a gate circuit belonging to a processor circuit. The opening of the gates is also made dependent on the output signals from the oscillator. If the pulses do not arrive at the gate circuit in a particular chronological sequence, they are rejected and an indication of non-authenticity of the inserted document is made. In the preferred exemplary embodiment of the known apparatus, the reading of the information in the document takes place while the coded card is stationary. Such static reading requires a very precise positioning of the document in its guide slot and any mechanical deformations or damage to the card must be prevented for correct operation. On the other hand, if a dynamic reader is used, the insertion speed must be precisely adjusted so as to prevent falsifications in the temporal occurrence of the test signals.
It is a further disadvantage of the known apparatus that only light of a specific wavelength may be used and that those regions which contain the test material must be transparent so that any soiling has to be avoided. Altogether, the known apparatus requires a substantial technical expenditure which is successful only if very particular conditions are met during the reading operation. In common usage, this is very often not the case so that the known apparatus is not sufficiently reliable and fool-proof in a very wide region of application.
It is further to be noted that information carriers, for example coded cards which contain invisible codes which are placed between only semi-transparent or weakly transparent cover sheets, require a substantial light intensity for correct reading. Accordingly, a suitable reader must have available to it a substantial amount of electric current, i.e., power. On the other hand, coded cards of this type are very popular due to their substantial resistance to forging and are widely used as credit cards, checking account cards and personal identification cards. All known opto-electronic readers for reading these cards employ the full operating power both during standby operation as well as during the reading operation and thus require considerable operating power at all times. They therefore must be supplied by a power source of substantially infinite capacity; for example, the commercial power grid, and the light sources normally employed, i.e., light-emitting photodiodes, are required to produce the maximum light intensity continuously. Accordingly, the known card readers are unsuitable for battery operation and, at the same time, have a relatively short operational lifetime.
Still another disadvantage of the known card readers is that they are usually provided with a mechanical card transport mechanism so as to guarantee a constant reading speed or pulse frequency during the admission of the card so that a well-defined coded bit stream of the stored data is obtained. These mechanical card transport mechanisms are subject to the common malfunctions of complicated mechanical systems.